The purpose is to determine the neurophysiological bases of phonatory control and its disorders. Acoustic analyses of phonatory signals are aimed at determining how phonation is altered by different pathologies. Initial studies compared groups of patients and found that jitter was increased in morphological changes in the vocal folds while neurological disturbances increased shimmer and not jitter. When individual data were examined to identify acoustic measures sensitive to different laryngeal pathologies, a great deal of overlap between normal and pathologic cases was found on jitter and shimmer. Patients with similar laryngeal disorders performed differently suggesting considerable variation between individuals in controlling laryngeal function. Acoustic measures have been developed to study the effects of involuntary vocal fold movements, observed fiberoptically, on phonation. Linear variations in frequency were associated with essential tremor of the vocal folds and were not found in other movement disorders. Further studies are planned to examine the acoustic effects of respiratory tremor, vocal tract tremor and vocal fold spasms. The results will assist in understanding neurophysiological control of phonation. In spasmodic dysphonia, an earlier study demonstrated that phonatory initiation was the speech task most affected in these patients. Subsequently, the coordination and timing between laryngeal and repiratory movements during phonatory initiation and whisper were examined. Slow reaction times and incoordination were found only during phonation, demonstrating task specific voluntary movement difficulties, commensurate with a focal dystonia. EMG studies of the intrinsic laryngeal muscles during speech tasks are aimed at determining the neurophysiological mechanisms underlying this movement disorder.